De-Extinction Defined

Passenger pigeon de-extinction aims to re-establish the ecological role of the species by introducing passenger pigeon traits into band-tailed pigeons. This is an iterative process that begins with identifying candidate passenger pigeon gene variants to edit into the genome of the band-tailed pigeon. This process happens computationally, or In Silico. How this project is designed during its early stages and how it is carried out downstream will have bearing on how de-extinction is defined and integrated into the tool box of conservation science.

Discovering the genetic blueprint of creating a passenger pigeon

De-extinction of the species starts with studying the genome and discovering what genes influence the passenger pigeon’s unique traits. Little is known about how gene pathways in any organism create phenotypes (observable traits such as color, shape, and behavior, to microscopic physiological functions). Some gene pathways and genomic regions have been associated with the focal traits of our project.

To further investigate the genomic sources of passenger pigeon traits our research team is applying an evolutionary approach – looking for “hot spots” of selection in the genome indicated by higher concentrations of differentiating mutations between band-tails and passenger pigeons.

How do we define a de-extinct passenger pigeon?

Our goal is not to recreate identical copies of historic passenger pigeons – this is an impossible thing to do. The DNA we can retrieve from passenger pigeon specimens is too fragmented to reassemble the entire genomic code – but we can map the sequence of genes and gene regulating regions that are most important to creating passenger pigeon traits.

Most mutations between the two species have simply built up over time – these “silent” mutations are not expressed, and for purposes of de-extinction can be considered “background noise.” To recreate the ecology of the passenger pigeon these mutations are useless. We want to introduce the smallest number of mutations necessary to recreate the passenger pigeon’s ecology.

What is a de-extinct passenger pigeon?

From an evolutionary standpoint, passenger pigeon de-extinction creates a new lineage of life: a lineage originating from the band-tailed pigeon but carrying the genes of the extinct passenger pigeon, very similar to hybridization. The difference is that we will introduce only gene mutations that change traits into the band-tailed pigeon lineage to create new passenger pigeons, ignoring silent mutations built up over millions of years of divergence. The difference between de-extinction and hybridization is that natural hybridization would introduce both trait mutations and silent mutations into a lineage randomly.

So what is the new passenger pigeon? Is it a hybrid? Is it a new species?

Technically this is an open debate – classifying species is a topic without consensus. But there is a suitable definition for what we aim to produce: an ecotype. An ecotype is a genetically distinct population of a species inhabiting a specific geographic range, which exhibits a uniquely adapted ecology. By introducing passenger pigeon genes to band-tailed pigeons we are facilitating their adaptation to take on the role of the passenger pigeon and inhabit the forests of eastern North America.

With this definition of a recreated passenger pigeons there are variations of success depending on how many traits are necessary to reproduce the disturbance generating flocks of the past for our forests of tomorrow.

Gradations of success

Passenger Pigeon images used in this diagram were modified from Tim Hough's original artwork

Passenger Pigeon images used in this diagram were modified from Tim Hough’s original artwork

High density band-tailed pigeon flocks that mimic those of passenger pigeons will reproduce passenger pigeon ecology.

Living in crowds isn’t only about behavior – hatchlings growing up rapidly meant that breeding passenger pigeons could leave nesting sites quickly before exhausting the food supply.

Male and female passenger pigeons looked different. This may not seem important to ecology, but how birds choose mates is very important to social living. The red breasts of males were likely a signal of superior fitness to females seeking the best mate in a crowd.

The signature graduated tail of the passenger pigeon was most likely also used in courtship displays and influenced mate choice. Tail shape also affects flight maneuverability –important when flying only a few inches from dozens of other birds. Although the bird above doesn’t look exactly like a passenger pigeon, it is de-extinction success.

The wing spots, blue slate body, black beak, and red feet of the passenger pigeon are beautiful, but probably did not affect the species’ ecology. Many of these traits may be associated with the major phenotypes we aim to reproduce – and it’s certainly the bird that the avid birder hopes to glimpse.